The present invention relates to a process for the preparation of linear copolymers containing segments of silsesquioxane and the resulting product, and more particularly to processes and polymers in which regularly alternating silsesquioxane and bridging group segments are produced.
Polysilsesquioxanes exhibit a number of potentially useful properties including high temperature stability in air and good adhesion to a number of substrates. Polysilsesquioxanes are also resistant to oxidation and degradation by ultraviolet light. They may find use as protective coatings for electronic devices and other substrates and as precursors for ceramic coatings, foams, fibers, and articles. However, polysilsesquioxanes also are problematic in that prior art synthesis routes either have produced low yields of product or are complex. Further, the resulting polymers, because of their highly crosslinked nature, have been difficult to handle, purify, and characterize. Polysilsesquioxanes also exhibit a well-known propensity to form insoluble, intractable gels.
The prior art includes several methods of synthesizing silsesquioxane-based polymers, (RSi(O).sub.1.5).sub.x. For example, Rahn et al, Mat. Res. Soc. Symp. Proc., (1990) v. 171:31-37, teach producing a silsesquioxane copolymer through a catalytic redistribution reaction followed by a reaction with alcohols. Laine et al, Chem. Mat., (1990), v. 2:464-472, teach the synthesis of methylsilsesquioxane polymers using a titanium-catalyzed redistribution of cyclomers or linear oligomers.
Sugiyama et al, U.S. Pat. No. 4,745,169, teach a polyorganosilsesquioxane polymer useful as a photoresist which is synthesized from a trichlorosilane or trialkoxysilane having a protected hydroxyl group. Kimura et al, U.S. Pat. No. 4,871,616, relate to a surface-treated polymethylsilsesquioxane powder used as an antiblocking agent. The polymer is formed by reacting a silicon compound with ammonia and water, allowing a hydrolysis/condensation reaction to occur, and then heating the resulting dispersion.
Linde et al, U.S. Pat. No. 5,043,789, describe a process for forming an insulating layer of a ladder-type silsesquioxane copolymer. The copolymer is synthesized by a condensation reaction with an aminoalkoxysilane and an arylalkoxysilane or arylsilazane monomer. Weidner et al, U.S. Pat. No. 5,047,492, describe several processes for the synthesis of organooligosilsesquioxanes using free radical addition and crosslinking reactions.
However, all of the prior art synthesis methods suffer from one or more of the following drawbacks. The synthesis routes used do not afford property control in the resulting polymer. The polymer quality and utility is limited due to impurities which arise from side reactions during synthesis. The synthesis route does not obtain a high yield, and/or the polymers produced have a limited shelf life because they contain reactive functionalities left over from the synthesis reaction.
Accordingly, the need still exists in the art for a synthesis process for polysilsesquioxanes which produces a high yield, tractable polymer which is essentially free of impurities and whose properties may be controlled.